The present invention relates to the optical determination of the physical position of tube ends in a steam generator of the type typically used in nuclear power stations.
The steam generators used in nuclear power stations typically have a tube sheet into which the ends of thousands of heat exchanger tubes are secured in a fluid tight relationship. Heated primary fluid from the nuclear reactor is passed through the tubes to exchange heat with the secondary working fluid which, in turn, drives the turbomachinery used to generate electricity. Since the primary fluid can be radioactive, the tube sheet is subject to inspection during those times that the power station is off-line for repairs and maintenance. Historically, the inspection of the tube sheet and the tube ends involved the use of technicians who would enter the steam generator and effect a visual inspection of the tube ends and conduct manual tests using various types of inspection devices to detect cracked, leaking, or otherwise damaged tubes. The defective tubes were typically "plugged" to seal the end of the tube to remove the affected tube from service and prevent leakage.
More recently, computer controlled robotic arms with specialized end-effectors have been used to effect the visual inspection of the tube ends and to effect the repairs. Contemporary robotic arms include several articulated joints with different length links between the various joints. Each joint includes, for example, a command-driven stepping motor or a rotary actuator and a cooperating angle position sensor (such as an optical encoder) that cooperate with a controller to control the angular movement of the links connected to the joint. The robotic arm is secured in place within the steam generator and beneath the tube sheet during the system shut-down. An end effector assembly is mounted to the end of the robotic arm and typically carries a television camera, one or more illumination sources, inspection tools (such as an eddy current device to detect cracked tubes), and/or tooling to effect plugging of a tube end. In general, the end effector is positioned in and moves in a plane spaced a selected distance (i.e., 2-5 cm or more) from the surface of the tube sheet.
The computer that controls the robotic arm is under the control of an operator who can move the end effector across the tube sheet using a simple joystick or by specifying a target x,y destination tube. Once the robotic arm is positionally calibrated or initialized at a known start position and knowing the tube diameter and the center-to-center pitch distance, the computer can calculate the best path from the start or initial position to the target x,y position and, in most cases, re-position the end-effector over the desired target tube. However, it is not uncommon for the robot arm to be subject to bending forces, especially when the arm is positioned at its maximum extent from its supports. Thus, an unintended deflection of one degree or so at an extension of two meters or so can cause the end effector to be positioned over a tube that is different from the target tube.
The mis-positioning of the end effector over a tube other than the intended target tube poses substantial safety implications. More specifically, the end effector can be unintentionally positioned over a known good tube and be commanded to plug that known good tube while inadvertently leaving an adjacent defective "target" tube in-service when the steam generator re-enters service.
Governmental agencies that regulate the nuclear industry require that the tube position be independently verified to maximize the probability that an end effector will be positioned over an intended target tube end. In general, tube ends can be independently counted by the system operator who counts tube ends as they move across a video monitor that displays the output of the TV camera mounted on the end effector. However, operator fatigue and inattention can cause a mis-count and the `loss` of the independent count.
Representative prior art disclosures of robotic arm end-effectors used in the servicing of steam generators in nuclear power plants include U.S. Pat. Nos. 5,751,610, 5,838,882, and 5,878,151, the disclosures of which are incorporated herein by reference.